Following this rule has generally made life easier for me -- in all senses of the phrase -- but I have come to the conclusion that one should add "and a light" to this axiom.
There are those who would argue that having a way to make fire with you pretty much takes care of the light situation, but in the
So. Some kind of flashlight is advisable.
For last year or so, I've been dinking around with a couple of portable light sources which have actually managed to impress me.
The first is the Photon Micro-light II with the white LED. This handy little darlin' clips onto your keychain slick as a whistle, is smaller than a quarter, and is bright enough to light a dark hallway for a decent distance.
Once it's on your key-chain, it pretty much goes unnoticed until such time as you need it, and you'd be surprised what kind of needs you can come up with. My brother and I have used Photon II's to signal each other across a mall parking lot, and I've locked mine in the 'ON' position, looped some fishing thread through the key-chain hole, and lowered it behind counters and into pipes looking for things.
The battery has been good for ~ a year, and is replaceable. At about $16.00 these things are a bargain.
While the microlight is a jolly decent bit of kit, there are times when one simply must have a bigger flashlight than the little Photon, and in this we are fortunate in that there is simply an un-ending parade of miniature lights to choose from.
The flashlights powered by lithium batteries are the current darlings of the lighting world, and for good reason: lithium batteries have a shelf-life of ten years or so, and the lights they power put out an amazing amount of light.
They do have a couple of minor drawbacks, though. While my Surefire 6P puts the sun to shame, when Surefire says that the batteries are good for about 60 minutes, they aren't kidding. That little Surefire runs like a top for the first 60 total minutes; at 60 minutes and thirty seconds the beam starts to get dim, and at 61 total minutes the batteries are dead and the light is out.
Also, running it for any extended period -- say more than 30 seconds -- and it gets HOT. One minute of constant 'on' gets you raise-a-blister temperatures.
Great for back-up to a regular flashlight, and great for searching rooms, but maybe not so good for every-day utility.
My little sister, bless her heart, gave me an iNova X5, as a present.
Folks, this little light is neater than kitten toes. It is powered with the same two lithium battery types that run my Surefire, but with five high-output white-light LED bulbs, the X5 is rated for 20 hours of total use, rather then the one hour total of the Surefire, and the X5 hasn't ever gotten warm enough to notice since I started using it.
It's been dropped in a swimming pool, dropped off of a house, and used in yawara drills without a whimper.
I wouldn't give up my Surefire for it -- the beam isn't anywhere as concentrated as the beam of the 6P, and the beam doesn't seem to have the same kind of range and distance as the 6P, but it's put-it-in-your-pocket-and-forget-about-it small, tough, and pretty much beats my old Mag-Light in every category except skull-thumping ability.
You may find something that suits you better -- and if you do, you should stick with it -- but if you're looking for compact, powerful little lights, you might cast an eye towards these two, they won't do you wrong.
*Obscure Terry Pratchett reference. Sorry about that.